Microscopic (Or atomic) diffusion

Abstract

Random motion of atoms is described in this chapter. The movement of a substance within a mixture by ‘random walk’ is governed by the kinetic energy within the system that may be affected by changes in concentration, pressure or temperature. It is impossible to observe the motion of individual atoms in solids, but diffusion occurs and may be observed. The concept of diffusion is shown by a schematic illustration of two planes, λ distance apart, showing the number of atoms n 1 and n 2 in each plane, respectively, and the corresponding fluxes, J 1 and J 2. The atoms diffuse by jumps and the frequency of jumps is related to the flux of atoms. By using the random walk for the diffusion it is possible to calculate the average distance that an atom covers from an initial reference point in all directions when the process is considered over a long period of time. It is shown that the average distance squared is related to the diffusion coefficient.